Workshop report. Linear-Scaling Ab Initio Calculations: Applications and Future Directions

The study of properties and of processes in materials, frequently hinges upon understanding phenomena which originate at the atomic level. In such cases the accurate description of the interactions between large numbers of atoms is critical and in turn requires the accurate description of the electrons which play a crucial role in the bonding of atoms into molecules, surfaces and solids. This can only be achieved by solving the equations of quantum mechanics. These equations are too complicated to solve exactly; however their solutions can be approximated by computational techniques. The most accurate ? but also most computationally demanding ? are the “ab initio” techniques which do not use any empirical adjustable parameters. Amongst them, the Density Functional Theory (DFT) formulation of quantum mechanics stands out as an excellent compromise between accuracy and computational efficiency. However, the applicability of ab initio techniques is severely limited by poor scaling: the computational effort needed to perform an ab initio calculation increases with (at least) the third power of the number of atoms, N. This cubic-scaling bottleneck limits the number of atoms we can study to a few hundred at most, even on parallel supercomputers. To overcome this length-scale limitation, a number of researchers worldwide have been pioneering the development of a novel class of ab initio methods with linear-scaling or “Order N” (O(N)) computational cost which nevertheless retain the same high level of accuracy as the conventional approaches. While physically motivated, such methods have proved particularly hard to develop as they introduce highly non-trivial localisation constraints. Nevertheless, many major obstacles have been overcome and a number of O(N) methods (SIESTA, CONQUEST, ONETEP, etc.) for ground state DFT calculations on systems with a gap (e.g. molecules, semiconductors and insulators) are now available and have reached a state of maturity that allows them to be used to study ”real” materials. The particular focus of this workshop is therefore to look forward to what can be achieved in the next few years. Our aim is twofold: (1) As O(N) methods are currently extending the applicability of DFT calculations to problems involving biomolecules and nanostructures they are leading to completely new levels of understanding of these systems. This CECAM meeting will give us the opportunity to make an appraisal of such large-scale simulations and their potential to connect more directly to experiments. (2) We also want to examine the options for extending linear-scaling to problems that cannot be treated by ground-state DFT but require other, more complex approaches. These include methods for treating metallic systems, excited states and wavefunction-based theories for including electronic correlation. Finding ways to transform these methods to linear-scaling cost, and hence extent their applicability to the nano-scale, is the next big challenge that the community of developers of large-scale electronic structure methods is beginning to face. We hope that this workshop will stimulate these major new O(N) methodological developments by bringing together the leading groups in the development of O(N) DFT methods with the leading groups in the development of metal and excited-state or wavefunction-based methods. Strong emphasis during the workshop will be given to discussion in order to promote the exchange of ideas between different communities (Physics, Chemistry, Materials Science, Biochemistry) which are all interested in large-scale applications with ab initio accuracy but are approaching them from different perspectives

Ab initio simulations of the interaction between water and defects on the calcite (101-4) surface

The interaction between water and calcite surfaces is relevant to a broad range of technological processes, but a fundamental understanding of the nature of the adsorbed water is still lacking. In an earlier publication we used density functional theory calculations to calculate the interaction between water and perfect (101Ì 4) calcite surfaces. Water was found to be strongly adsorbed as associated molecules. In this paper water adsorption on (101Ì 4) calcite surfaces with steps and vacancies is investigated. A water molecule was found to bind more strongly to acute steps than to obtuse steps. The lowest energy position was found to be the base of the step for acute steps and on top of the step for obtuse steps. Water molecules were found to exhibit very strong binding to surface vacancies. Associative adsorption was favored near cation vacancies; however, the water was found to dissociate, to form a bicarbonate ion and a hydroxide ion, near anion vacancies. © 2010 American Chemical Society

Family refusal of eye tissue donation from potential solid organ donors: a retrospective analysis of summary and free-text data from the UK National Health Service Blood and Transplant Services (NHS-BT) National Referral Centre (1 April 2014 to 31 March 2017)

Objectives Long-standing undersupply of eye tissue exists both in the UK and globally, and the UK National Health Service Blood and Transplant Service (NHSBT) has called for further research exploring barriers to eye donation. This study aims to: (1) describe reported reasons for non-donation of eye tissue from solid organ donors in the UK between 1 April 2014 and 31 March 2017 and (2) discuss these findings with respect to existing theories relating to non-donation of eyes by family members. Design Secondary analysis of a national primary data set of recorded reasons for non-donation of eyes from 2790 potential solid organ donors. Data analysis including descriptive statistics and qualitative content analysis of free-text data for 126 recorded cases of family decline of eye donation. Setting National data set covering solid organ donation (secondary care). Participants 2790 potential organ donors were assessed for eye donation eligibility between 1 April 2014 and 31 March 2017. Results Reasons for non-retrieval of eyes were recorded as: family wishes (n=1339, 48% of total cases); medical reasons (n=841, 30%); deceased wishes (n=180, 7%). In &gt;50% of recorded cases, reasons for non-donation were based on family's knowledge of the deceased wishes, their perception of the deceased wishes and specific concerns regarding processes or effects of eye donation (for the deceased body). Findings are discussed with respect to the existing theoretical perspectives. Conclusion Eye donation involves distinct psychological and sociocultural factors for families and HCPs that have not been fully explored in research or integrated into service design. We propose areas for future research and service development including potential of only retrieving corneal discs as opposed to full eyes to reduce disfigurement concerns; public education regarding donation processes; exploration of how request processes potentially influence acceptance of eye donation; procedures for assessment of familial responses to information provided during consent conversations.</p

Behind the scenes: The corporate governance preferences of institutional investors

We survey institutional investors to better understand their role in the corporate governance of firms. Consistent with a number of theories, we document widespread behind-the-scenes intervention as well as governance-motivated exit. These governance mechanisms are viewed as complementary devices, with intervention typically occurring prior to a potential exit. We further find that long-term investors and investors that are less concerned about stock liquidity intervene more intensively. Finally, we find that most investors use proxy advisors and believe that the information provided by such advisors improves their own voting decisions